Fuel storage tanks, such as shallow tanks commonly used to hold fuel for a backup electrical generator, include a relief vent assembly that allows pressurized fluid to escape from the tank when a buildup of pressure within the tank exceeds a selected level that is greater than the pressure outside the tank. Often, these tanks are located above the ground and frequently include an inner tank that holds the fuel and an outer tank that acts as a safety vessel by containing the fuel from the inner tank should the structure of the inner tank be compromised. As such tanks normally hold fuel in liquid form, they tend not to be designed to hold a highly pressurized fluid. Thus, the relief vent assembly allows a buildup of pressurized fluid, when such condition arises, to discharge from the tank and reduce stress on the structure of the tank.
Typically, such an increase in pressure within the tank is due to an increase in heat in the area surrounding the tank, often by a fire. In order to prevent damage from rapid expansion of the pressurized fluid pushing against the structure of the tank, the vent assembly must allow this pressurized fluid to be discharged safely from the tank. Preferably, this venting occurs at a flow rate high enough to ensure that the pressure within the tank can be maintained at a manageable level.
Inasmuch as the vent assembly includes a passage from inside the tank to the outside atmosphere, a lid is customarily included such that, under normal operating conditions, the lid keeps the vent closed and maintains the inside of the tank substantially closed off from the atmosphere for various reasons. Accordingly, in addition to allowing pressurized fluid to vent out from the tank, the vent assembly should also prevent foreign matter from entering the tank. A short stroke lid is often used to accommodate the frequently limited space constraints around the fuel tank as well as to prevent foreign matter from entering the tank, including such things as contaminants or unauthorized devices used to syphon fuel from the tank.
Conventionally, a gravity closing vent has been used with these types of fuel tank systems. Such a vent provides satisfactory performance in some respects, although those of ordinary skill in the art will recognize that a gravity closing lid must be in a vertical orientation in order to function properly, limiting design options for placement on a tank. Additionally, prior art gravity closing vent assemblies have presented a stepped bore with a large and axially centrally located crossbar for supporting the lid. This crossbar introduces turbulence to the flow of the fluid in the chamber of the vent before the fluid has passed substantially through the vent, causing the fluid flow to slow down and limiting the maximum flow rate through the vent and out of the tank. Slowed flow rate is undesirable, as it can impede evacuation of pressurized fluid from the tank during emergency vent situations.
High flow rates of the pressurized fluid are advantageous, as a quick evacuation of pressurized fluid is desired in emergency situations. Furthermore, the popular certification organization Underwriters Laboratories (“UL”) requires, in order to receive certification, that a vent be able to provide a flow rate of 110,000 cubic feet per hour at two and a half pounds per square inch of pressure for a four inch open pipe. Some prior art gravity closing vents have been unable to meet this threshold and, consequently, have not qualified for a commercially advantageous UL certification.